Arm extension machine

ABSTRACT

The subject invention provides a triceps exercise machine which minimizes the torque on a user&#39;s shoulders throughout the range of motion. The exercise machine applies a pulling force to the user&#39;s arms, the magnitude and direct of which remains substantially constant, such that as the users&#39; hands rotate about the user&#39;s elbow the effective toque on the user&#39;s elbows and shoulders remains substantially constant.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is related to and claims priority to U.S. Provisional Patent Application Serial No. 60/361,469, filed Mar. 4, 2002, entitled ARM EXTENSION MACHINE, the entirety of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] n/a

FIELD OF THE INVENTION

[0003] The present invention relates to the field of exercise and physical rehabilitation equipment, and in particular, to an apparatus for exercising the arms and triceps muscles.

BACKGROUND OF THE INVENTION

[0004] It is often necessary or desirable for a person to exercise a particular muscle or muscle group such as to recover from surgery or injury, to increase strength, stamina and endurance, and to maintain an active and healthy lifestyle, as well for aesthetic purposes. Various machines have been developed to exercise different muscles and muscle groups by forcing the muscles to contract and extend under a load, such as by moving a free weight against the force of gravity or by moving an object whose movement is resisted by resistance force, such as a spring or a weight attached to a cable and pulley system.

[0005] One exercise device that has been developed to exercise the triceps muscles is known as the arm extension. The arm extension stresses the muscles of the posterior side of the upper arm, such as the triceps brachii, as well as several ancillary muscles groups in the arm and forearm. In executing an arm extension movement, a person starts with his or her arm flexed at the elbow, usually exceeding ninety degrees of flexion, and grips an object with the hand. The arm is then extended by rotating the lower arm around the elbow pivot, while a load force acts on the object, such as a free weight under gravity, or a resistance force coupled to the object. The arm acts against such a force throughout the motion, from the elbows-flexed position up to the elbows-extended position wherein the arms are relatively straight.

[0006] To be effective, the arm extension should focus and isolate only the triceps and upper arm muscles as much as possible. Known machines and devices which use this motion however, often generate unwanted stresses and torques on other body parts. This may force the exercise to end prematurely and/or reduce the ability to maximally train the target muscle group.

[0007] For example, as shown in FIG. 1, when executing an arm extension motion on a typical training device, the load is applied through an input arm that has its pivot aligned with the elbow. This creates a load, which is applied through the hands and is substantially perpendicular to the forearm throughout the entire range. At the beginning of the exercise when the elbow is close to ninety degrees of flexion, the load vector is typically creating an equal amount of torque at the shoulder and the elbow. At the end of the exercise, when the user's arm is at full extension, since the load vector is now being applied to the full length of the users arm, the torque at the shoulder would typically be double of the torque applied to the elbow. Since it is difficult for the shoulder to control this magnitude of load, the result is that the elbow usually lifts off the support pad and the exercise is not able to be performed. The solution has been to use a lower weight which the shoulder can handle, but this defeats the purpose of the exercise which is to apply torque to the elbow for the purpose of exercising the triceps muscles.

[0008] It is desirable therefore to provide an arm extension machine that more effectively isolates the triceps muscles throughout the course of arm extension. It is also desirable to provide an arm extension machine which minimizes the torque and stress on the shoulder joint while a load is applied to the arms during the course of arm extension.

SUMMARY OF THE INVENTION

[0009] The subject invention provides a triceps exercise machine which minimizes the torque on a user's shoulders throughout the range of motion, while maximizing the load at the triceps. The exercise machine applies a pulling force to the user's arms, the magnitude and direction of which remains substantially constant, such that as the users' hands rotate about the user's elbows the effective torque on the user's elbows is maintained, without a subsequent increase in torque at the shoulders.

[0010] The exercise machine of the subject invention includes a frame and a user support structure mounted to the frame. An input assembly is mounted to the frame behind the user support structure. The input assembly imparts an effective torque on the user's shoulders, the effective torque increasing, then decreasing through the range of motion in unison with the normal strength capacity of the individual.

[0011] The input assembly includes a linkage assembly pivotally connected to the frame, a pair of trailing arms pivotal connected to the linkage assembly, and a pair of handles connected to the pair of trailing arms opposite that the linkage assembly, such that the handles are positioned in front of the user support structure, above the user's shoulders. A weight resistance mechanism is operably connected to the linkage assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

[0013]FIG. 1 is a schematic representation of a prior art exercise machine;

[0014]FIG. 2 is a schematic representation of the exercise machine of the present invention;

[0015]FIG. 3 is a front perspective view of an exemplary embodiment of the present invention;

[0016]FIG. 4 is a back view of the embodiment of FIG. 3;

[0017]FIG. 5 is a left side view of the embodiment of FIG. 3;

[0018]FIG. 6 is a right side view of the embodiment of FIG. 3;

[0019]FIG. 7 is a front perspective view of an alternative embodiment of the present invention;

[0020]FIG. 8 is a right side view of the embodiment of FIG. 7; and

[0021]FIG. 9 is top view of the embodiment of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The subject invention provides a triceps exercise machine which minimizes the torque on a user's shoulders throughout the range of motion. As shown in FIG. 2, the exercise machine applies a pulling force to the user's arms, as indicated by force vector “F.” The effective torque applies to the user's elbows and shoulders is the force vector “F” multiplied by the perpendicular distance between the user's elbows and shoulders and the force vector “F,” namely L₁ and L₂ respectively. The user's shoulders will generally be disposed below the user's elbows, such that in the start position L₂ is slightly greater than L₁. Thus, the initial torque on the user's shoulders is generally close in magnitude to the torque on the user's elbows. Throughout the range of motion the magnitude and direction of the force vector “F” remains substantially constant, such that as the users' hands rotate about the user's elbows, the perpendicular distances L₁ and L₂ increase slightly then decrease, decreasing the effective torque on the user's elbows and shoulders at the finish.

[0023] In an exemplary embodiment, as shown in FIG. 3, the exercise machine 10 of the present invention includes a support frame 12 having a front leg 14 and rear base 16. A seat 18 is mounted to the front leg 14 of the support frame 12. The seat 18 is adapted to be positioned at various heights along the front leg 14 to provide a comfortable starting position and the proper alignment of the shoulder for users of varying stature. A backrest 20 is mounted on the front leg 14 above the seat 18, wherein the backrest 20 defines a plane. Although this machine is shown to have a minimally inclined backrest, other configurations provide various degrees of incline. As shown, the front leg 14 angles away behind the seat 18 in an upward direction such that the backrest 20 is in a partially reclined position. The seat 18 and backrest 20 comprise a user support structure adapted to maintain the user in a comfortable position for exercising. Two elbow supports 24 are pivotally mounted to the front leg 14 behind the backrest 20. Each of the elbow supports 24 includes an elbow support pad 26, where the elbow supports 24 are pivotable about the front leg 14 for engagement of the elbow support pads 26 by a user's elbows.

[0024] As shown in FIGS. 3 and 4, an input assembly 30 is positioned behind the seat 18, and pivotally mounted to the support frame 12 for rotation about a horizontal axis A-A. The input assembly 30 includes a horizontal tube 32 pivotally mounted to the support frame 12. A pair of lower vertical supports 34 are affixed to the horizontal tube 32 at their distal ends and a horizontal support 36 is affixed to the vertical support's 34 proximal ends, where the horizontal support 36 is parallel to the horizontal tube 32. A pair of upper vertical supports 38 are affixed to the horizontal support 36 at their distal end, and include a pair of trailing arms 40 pivotally mounted to the upper vertical support's 38 proximal ends. The trailing arms 40 each include a handle 42, the handles 42 being positioned substantially in front of the backrest 20. The handles 42 are pivotally connected to the trailing arms 40, enabling the handles 42 to remain substantially perpendicular to the longitudinal axis of the user's forearm throughout the range of motion with minimum stress to the user's wrist. The pivotal connection of the handles 42 enables a user to maintain the proper biomechanical alignment of the user's wrist and forearm during performance of the exercise. A pair of gas springs 44 are interposed between the upper vertical supports 38 and the trailing arms 40, such that the trailing arms 40 are supported when the exercise machine 10 is not in use.

[0025] In an exemplary embodiment, as shown in FIGS. 3 and 4 a weight stack frame 48 is attached to the support frame 12 by beams 52 a and 52 b, such that the weight stack 50 is easily accessed by a user seated in the seat 18. Chevron-shaped bridges 54 a and 54 b are rigidly mounted to the support frame 12 and the weight stack brace 48, respectively. (See also FIG. 5) The chevron-shaped bridges 54 a and 54 b support a transmission 56, including a shaft 58, a first cam 60, and a second cam 62.

[0026] As shown in FIG. 5, a weight stack pulley set 64 a and 64 b is mounted to the top of the weight stack frame 48, with pulley 64 a aligned with the first cam 60 and pulley 64 b aligned with the weight stack 50. Rails 66 are mounted vertically within the weight stack frame 48. Weight stack 50 is slidingly mounted to the rails 66 and provides a resistance to the exercise.

[0027] In alternative embodiments, other mechanisms for providing resistance, such as friction fitting, springs, elastic bands, pneumatic, hydraulic, electromagnetic resistance, or an air resistance fan could be employed (either alone or in combination) and still practice the invention. Additionally, free weights could be operably engaged to the trailing arms 40 to resist the movement.

[0028] As shown in FIGS. 5 and 6, the input assembly 30 is operably connected to the weight stack 50 via the transmission system 56. A frame pulley 68 is mounted to the rear base 16 of the support frame 12, above and aligned with the second cam 62. A first cable 70 is threaded from the second cam 62 about the frame pulley 68 and connecting to a vertical support 34, such that the second cam 62 is caused to rotate when the handles 42 are extended.

[0029] A belt 72 is attached at one end to the first cam 60 and extends over the weight stack pulleys 64 a and 64 b and is attached to the weight stack 50. As the user pulls forward on the handles 42 the cable 70 unwinds, rotating the second cam 62. As the second cam 62 rotates, the shaft 58 and the first cam 60 rotate as well. (See also FIG. 4). The rotation of the first cam 60 pulls the belt 72 over the weight stack pulleys 64 a and 64 b, and thus lifts the weight stack 50.

[0030] In a method of use, a user adjusts the seat 18 position, such that the user's upper arm is substantially perpendicular to the backrest 20. The user sits onto seat 18 and rests his/her back against the backrest 20, facing generally in the direction “D” as shown. The user's upper arms are placed onto elbow support pads 26, the arms being bent and the hands gripping the handles 42. The user then executes the arm extension movement by pulling the handles 42 and trailing arms 40 generally in the direction “D”. As the handles 42 are pulled generally in direction “D,” the trailing arms 40 move in the direction “D,” forcing the tube 32 to rotate about the horizontal axis A-A and imparting a pulling force on the cable 70, rotating the second cam 62. As the second cam 62 rotates, the shaft 58 and the first cam 60 rotate as well. The rotation of the first cam 60 pulls the belt 72 over the weight stack pulleys 64 a and 64 b, and thus lifts the weight stack 50. The foregoing is a description of a “positive” direction for the user as the user's arms are extended, exercising the triceps muscles. When the user's arms are thereafter retracted to the elbows-flexed position, the handles 42 return to their starting position in a direction generally opposite to D, wherein the handles 42 move in the direction “N” as shown, denoting a “negative” direction for the user.

[0031] In either movement direction, the load is always in the same direction so as to pull the users hand back to the start position (elbows flexed).

[0032] As shown in FIG. 6, the force vector generated on the user's arms as the handles 42 are pushed in direction “D” is a reaction force “F” generally opposite direction “D” and equal in magnitude to the effective load provided by the input assembly 30 and cables linked to weight stack 50. This force vector “F,” when multiplied by length “L₁” equals the torque on a user's elbow resting on the elbow support pad 28, the point of rotation being generally coincident with the elbow joint at around point “X”. The user's shoulder joints will generally be disposed at around point “Y” as shown, such that the effective torque on the shoulders will be equal to “F” multiplied by the length “L₂” as shown. Since the shoulders are only minimally lower than the elbows when a user is seated and executing the arm extension motion in machine 10, “L₂” is generally only slightly longer than “L₁”. Thus, the torque on the shoulders is generally substantially similar to the torque on the elbows.

[0033] In an alternative embodiment, as shown in FIG. 7, the exercise machine 100 of the present invention includes a support frame 102 having a front leg 104 and rear base 106. A seat 108 is mounted to the front leg 104 of the support frame 102. The seat 108 is adapted to be positioned at various heights along the front leg 104 to provide a comfortable starting position and the proper alignment of the shoulders for users of varying stature. A backrest 110 is mounted on the front leg 104 above the seat 108, wherein the backrest 110. The seat 108 and the backrest 110 each define a common plane of symmetry, the plane of symmetry being perpendicular to the ground. Although this machine is shown to have a minimally inclined backrest, other configurations provide various degrees of incline. As shown, the front leg 104 angles away behind the seat 108 in an upward direction such that the backrest 110 is in a partially reclined position. The seat 108 and backrest 110 comprise a user support adapted to maintain the user in a comfortable position for exercising. Two elbow supports 112 are pivotally mounted to the front leg 104 behind the backrest 110. Each of the elbow supports 112 includes an elbow support pad 114, where the elbow supports 112 are pivotable about the front leg 104 for engagement of the elbow support pads 114 by a user's elbows.

[0034] As shown in FIGS. 7 and 8, an input assembly 118 is positioned behind the seat 108, and includes a pair of trailing arms 120 each having a handle 122 at the proximal ends and pivotally connected to a main linkage bar 124 at the distal ends. The handles 122 are positioned substantially in front of the backrest 110 and are pivotally connected to the trailing arms 120 such that the handles 122 will rotate. This configuration enables the handles 122 to remain substantially perpendicular to the longitudinal axis of the user's forearm throughout the range of motion with minimum stress to the user's wrist, maintaining the proper biomechanical alignment of the user's wrist and forearm during performance of the exercise.

[0035] The main linkage bar 124 is pivotally connected to the support frame 102, such that as the handles 122 are extended, the main linkage bar 124 rotates about horizontal axis B-B. A pair of trailing arm supports 126 are affixed to the front leg 104, behind the backrest 110, to support the trailing arms 120 when the exercise machine 100 is not in use.

[0036] A shown in FIGS. 7 and 9, the trailing arms 120 are rigid elongate members pivotally connected to the main linkage bar 124 such that the trailing arms 120 are rotatable about dual axes. The proximal end of the main linkage bar 124 includes a horizontal pivot bar 128, defining a horizontal axis C-C. The distal ends of the trailing arms 120 are pivotally connected to the horizontal pivot bar 128, such that the trailing arms 120 pivot about the horizontal axis C-C as the handles 122 are extended. Additionally, the trailing arms 120 include a cross-axle pivot 130, defining axes P-P substantially perpendicular to the horizontal axis C-C. The cross-axle pivot 130 enables the distance between the handles 122 to be increased or decreased, to fit the size of the user.

[0037] A weight stack brace 132 is attached to the support frame 102 by beams 134 a and 134 b, such that the weight stack 136 is easily accessed by a user seated in the seat 108. Chevron-shaped bridges 138 a and 138 b are rigidly mounted to the support frame 102 and the weight stack brace 132, respectively. The chevron-shaped bridges 138 a and 138 b support a transmission system 140, including a shaft 142, a first cam 144, and a second cam 146. A weight stack pulley set 148 a and 148 b is mounted to the top of the weight stack brace 132, with pulley 148 a aligned with the first cam 144 and pulley 148 b aligned with the weight stack 136. Rails 150 are mounted vertically within the weight stack brace 132. Weight stack 136 is slidingly mounted to the rails 150 and provides a resistance to the exercise.

[0038] In alternative embodiments, other mechanisms for providing resistance, such as friction fitting, springs, elastic bands, pneumatic, hydraulic, electromagnetic resistance, or an air resistance fan could be employed (either alone or in combination) and still practice the invention. Additionally, free weights could be operably engaged to the trailing arms 120 to resist the movement.

[0039] As shown in FIGS. 8 and 9, the input assembly 118 is operably connected to the weight stack 136 via the transmission system 140. A frame pulley 152 is mounted to the rear base 106 of the support frame 102, above and aligned with the second cam 146. A first cable 154 is threaded from the second cam 146 about the frame pulley 152 and connects to a secondary linkage bar 156, where the secondary linkage bar 156 is pivotally connected to the support frame 102. A cross linkage bar 158 is interposed between and pivotally connected to the secondary linkage bar 156 and the main linkage bar 124, such that the second cam 146 is caused to rotate when the handles 122 are extended.

[0040] A belt 160 is attached at one end to the first cam 144 and extends over the weight stack pulleys 148 a and 148 b and is attached to the weight stack 136. As the user pulls forward on the handles 122 the cable 154 unwinds, rotating the second cam 146. As the second cam 146 rotates, the shaft 142 and the first cam 144 rotate as well. The rotation of the first cam 144 pulls the belt 160 over the weight stack pulleys 148 a and 148 b, and thus lifts the weight stack 136.

[0041] As shown in FIG. 8, the force vector generated on the users arms as the handles are pushed in direction D is a reaction force “F” generally opposite direction “D” and equal in magnitude to the effective load provided by the assembly of the trailing arms 120, main linkage bars 124, and transmission 118 linked to weight stack 136. This force vector “F,” when multiplied by length “L₁” equals the torque on a users elbow resting on the elbow support pad 122, the point of rotation being generally coincident with the elbow joint at around point “X”. The user's shoulder joint will generally be disposed at around point “Y” as shown, such that the effective torque on the shoulder will be equal to “F” multiplied by the length “L₂” as shown. Since the shoulders are only minimally lower than the elbows when a user is seated and executing the arm extension motion in the exercise machine 100, “L₂” is generally only slightly longer than “L₁.” Thus, the torque on the shoulders is substantially similar to the torque on the elbows.

[0042] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims. 

What is claimed is:
 1. An exercise machine for exercising a user's upper arm muscles comprising: a frame; and an input assembly pivotally connected to the frame and engagable by the user, the input assembly imparting an effective torque on the user's shoulders, the effective torque on the user's shoulders remaining substantially constant through the range of motion.
 2. The exercise machine according to claim 1, wherein the input assembly imparts an effective torque on the user's elbows, the effective torque on the user's elbows remaining substantially constant through the range of motion.
 3. The exercise machine according to claim 2, wherein the effective torque on the user's shoulders is substantially similar to the effective torque on the user's elbows.
 4. The exercise machine according to claim 1, wherein the input assembly comprises a linkage assembly pivotally connected to the frame, a pair of trailing arms pivotal connected to the linkage assembly, and a pair of handles connected to the pair of trailing arms opposite the linkage assembly, such that the handles are positioned above the user's shoulders.
 5. The exercise machine according to claim 4, wherein handles are pivotally connected to the trailing arms, such that the user's hands remain substantially perpendicular to the user's forearms through the range of motion.
 6. The exercise machine according to claim 4, wherein the linkage assembly comprises at least one vertical support member pivotally connected to the frame, wherein the pair of trailing arms are pivotal connected to the at least on vertical support member opposite the frame.
 7. The exercise machine according to claim 4, wherein the linkage assembly comprises: a horizontal shaft pivotally connected to the frame and rotatable about a horizontal axis; at least one lower vertical support extending from the horizontal shaft; a horizontal support affixed to the pair of lower vertical supports, opposite the horizontal shaft; and at least one upper vertical support affixed to the horizontal support, wherein the pair of trailing arms are pivotally attached to the upper vertical support opposite the horizontal support.
 8. The exercise machine according to claim 4, wherein the linkage assembly comprises: a main linkage bar comprising a first end and a second end, wherein the first end is pivotally connected to the frame and the second end includes a horizontal pivot bar defining a horizontal axis and a pair of hinge mechanism each defining a pivot axis substantially perpendicular to the horizontal axis, wherein the trailing arms are affixed to the hinge mechanisms, such that the trailing arms are rotatable about the horizontal pivot bar's horizontal axis and about the hinge mechanism's perpendicular axis; a secondary linkage bar pivotal connected to the frame; and a cross linkage bar pivotally interposed between the main linkage bar and the corresponding pivot axis.
 9. The exercise machine according to claim 1, further comprising a user support structure including: a seat mounted to the frame; a backrest mounted to the frame above and behind the seat; and a pair of elbow supports pivotally connected to the frame behind the backrest, each including an elbow pad such that the elbow pads are positionable in front of the backrest.
 10. The exercise machine according to claim 1, further comprising a resistance mechanism operably connected to the input assembly.
 11. An exercise machine for exercising a user's upper arm muscles comprising: a frame; an input assembly including a linkage assembly pivotally connected to the frame, a pair of trailing arms pivotally connected to the linkage assembly, and a pair of handles connected to the pair of trailing arms opposite that the linkage assembly, such that the handles are positioned above the user's shoulders; and a resistance mechanism operably connected to the input assembly.
 12. The exercise machine according to claim 11, wherein the input assembly imparts an effective torque on the user's shoulders, the effective torque on the user's shoulders remaining substantially constant through the range of motion.
 13. The exercise machine according to claim 12, wherein the input assembly imparts an effective torque on the user's elbows, the effective torque on the user's elbows remaining substantially constant through the range of motion.
 14. The exercise machine according to claim 13, wherein the effective torque on the user's shoulders is substantially similar to the effective torque on the user's elbows.
 15. The exercise machine according to claim 11, further comprising a user support structure having: a seat mounted to the frame; a backrest mounted to the frame above and behind the seat; and a pair of elbow supports pivotally connected to the frame behind the backrest, each including an elbow pad such that the elbow pads are positionable in front the backrest.
 16. The exercise machine according to claim 11, wherein handles are pivotally connected to the trailing arms, such that the user's hands remain substantially perpendicular to the user's forearms through the range of motion.
 17. The exercise machine according to claim 11, wherein the linkage assembly comprises at least one vertical support member pivotally connected to the frame, wherein the pair of trailing arms are pivotal connected to the at least on vertical support member opposite the frame.
 18. The exercise machine according to claim 11, wherein the linkage assembly comprises: a horizontal shaft pivotally connected to the frame and rotatable about a horizontal axis; at least one lower vertical support extending from the horizontal shaft; at horizontal support affixed to the pair of lower vertical supports, opposite the horizontal shaft; and a at least one upper vertical support affixed to the horizontal support, wherein the pair of trailing arms are pivotally attached to the upper vertical support opposite the horizontal support.
 19. The exercise machine according to claim 11, wherein the linkage assembly comprises: a main linkage bar comprising a first end and a second end, wherein the first end is pivotally connected to the frame and the second end includes a horizontal pivot bar defining a horizontal axis and a pair of hinge mechanism each defining a pivot axis substantially perpendicular to the horizontal axis, wherein the trailing arms are affixed to the hinge mechanisms, such that the trailing arms are rotatable about the horizontal pivot bar's horizontal axis and about the corresponding pivot axis; a secondary linkage bar, pivotally connected to the frame; and a cross linkage bar pivotally interposed between the main linkage bar and the secondary linkage bar.
 20. An arm extension machine for exercising the upper arm muscles, comprising: a frame disposed to be placed on a surface, a load mechanism providing a load, a seat and a backrest coupled to the frame, each defining a common plane of symmetry, the plane of symmetry being perpendicular to the surface, first and second arms coupled to the frame and oriented substantially parallel to the plane of symmetry and the surface, each having a proximal and a distal end portion, and a handle disposed at the proximal end portion, first and second linkages, each linkage having a first pivot and a second pivot, the distal end portion of the first arm being rotatably coupled to the first linkage to rotate the first arm about the first pivot of the first linkage to define a first plane of motion, and to rotate the first arm about the second pivot of the first linkage to define a second plane of motion, the first plane of motion being substantially perpendicular to the surface and parallel to the plane of symmetry, the second plane of motion being substantially parallel to the surface and perpendicular to the plane of symmetry, and the distal end portion of the second arm being rotatably coupled to the second linkage to rotate the second arm about the first pivot of the second linkage to define a third plane of motion, and to rotate the second arm about the second pivot of the second linkage to define a fourth plane of motion, the third plane of motion being substantially perpendicular to the surface and parallel to the plane of symmetry, the fourth plane of motion being substantially parallel to the surface and perpendicular to the plane of symmetry. 